The immune system of a mammal develops during gestation and becomes active in the late mammalian fetus. Although active, it still might be characterized as ‘immature’ because it has not been challenged to any significant extent by antigens; the fetus is largely protected from antigens by the mother. This ‘immature’ immune system, however, is supplemented by the transfer of maternal immunoglobulin to the fetus (or in some cases to the neonate) to provide humoral immunity during the first weeks of independent life.
Rats and mice receive most maternal immunoglobulin G (IgG) by suckling from colostrum and milk, although some is acquired prenatally. Cattle also receive IgG from colostrum. In rabbits, IgG is transported to the fetus across the yolk sac. Little is know about the transfer of IgG to the fetus or neonate in humans. Most evidence suggests that human mothers transfer humoral immunity to an offspring only before birth, although IgA transferred to a neonate via breast milk is believed to play a role in protecting the neonate against enteric infection.
The delivery of maternal IgG to the mammalian fetus and/or neonate requires transport across an epithelial barrier which is largely impervious to macromolecules. The transport of macromolecules across such an epithelial barrier may occur by non-specific and specific receptor-mediated mechanisms. Receptor non-specific mechanisms are represented by paracellular sieving events, the efficiency of which are inversely related to the molecular weight of the transported molecule. Transport of macromolecules such as IgG across this paracellular pathway is highly inefficient. Descriptions of receptor-mediated transport of immunoglobulins through intestinal epithelial cells are limited thus far to the polymeric immunoglobulin receptor and the enterocyte receptor of IgG (a major histocompatibility complex (MHC) class I related Fc receptor). These two receptor systems differ in their specificity for immunoglobulin isotype, in their direction of immunoglobulin transport across the epithelial cell and in their tissue-specific expression. Both may play a role in molding the immature immune system.
The polymeric immunoglobulin receptor is expressed on the basolateral surfaces of enterocytes, hepatocytes and/or biliary duct epithelial cells. It transports polymeric IgA and IgM to the apical (luminal) surfaces, concentrating these immunoglobulins for antimicrobial defense and antigen exclusion.
The enterocyte receptor for IgG, which has homology to the MHC class I heavy chain and is associated with beta2-microglobulin (β2M), is expressed on neonatal enterocytes of the rat and mouse. IgG is transported transcellularly in a luminal to serosal direction across the intestinal epithelium of these rodent neonates. On the apical surface of the enterocyte, the Fc portion of IgG is bound to the enterocyte receptor at the relatively acidic pH of the lumen (about pH 6.0). Following transcytosis to the basolateral plasma membrane, discharge of the immunoglobulin occurs at the relatively neutral pH of the interstitial fluids (about pH 7.4). The rodent neonatal Fc receptor (FcRn) therefore could be responsible for delivery of maternal IgG to the neonate and as such may be responsible for the passive acquisition of IgG during this period.
In humans, maternal IgG is actively transported across the placenta. The receptor responsible for this transport has been sought for many years. Several IgG-binding proteins have been isolated from placenta. FcγRII was detected in placental endothelium and FcγRIII in syncytiotrophoblasts. Both of these receptors, however, showed a relatively low affinity for monomeric IgG. Recently, the isolation from placenta of a cDNA encoding a human homolog of the rat and mouse enterocyte receptor for IgG was reported. (Story, C. M. et al., J. Exp. Med., Vol. 180:2377–2381, December 1994) The complete nucleotide and deduced amino acid sequence is reported. This Fc receptor for IgG may be responsible for the transport of maternal IgG to the human fetus (and even possibly to the neonate), as the molecule is highly homologous over its open reading frame with the rat FcRn sequence (69% nucleotide identity and 65% predicted amino acid identity). So called passive immunization in the human fetus (and possibly in the human neonate) now may become better understood.
In contrast to passive immunization which involves supplementing a host's immune system with antibodies derived from another, active immunization involves stimulation of the host's own immune system to generate in vivo the desired immune response. The most widely practiced methods of active immunization in children and adults involve injections of an immunogen, once as an initial dose and then at least once again as a booster dose. These methods suffer many serious drawbacks, including the risks associated with the use of needles that can transmit diseases such as AIDS and hepatitis. (When tolerizing a patient against an allergen, the problems are compounded in that repeated injections over a long period of time often are required.) These methods also do not necessarily trigger adequately the first line of defense against many pathogens, that is, mucosal immunity. Mucous membranes line the airways, the reproductive system and the gastrointestinal tract, and this mucosal surface represents the first portal of entry for many diseases. An oral vaccine that is easy to deliver and that triggers mucosal immunity would be highly desirable.
Immunization using oral vaccines is problematic. Often little or no immune response is achieved. To enhance the immune response, antigens of interest have been coupled to carriers that are known to be strongly immunogenic. For example, researchers have delivered antigens using Bacille Calmette-Guérin (BCG) as a carrier; BCG is a bacterium originally used as an oral vaccine against tuberculosis. A problem with such carriers is that the patient will develop antibodies against the carrier itself, which can be troublesome if the carrier is used again for delivering a different antigen to the same patient. To date, no general strategy for oral vaccines has proven successful.
Immunoglobulin and portions thereof in the past have been conjugated to drugs and imaging agents to target and destroy cell populations and to extend the half-lives of certain agents. Immunotoxins are an example of such conjugates. Such conjugates, however, have never been proposed as useful for initiating an immune response.
A small body of work has focused on the tolerogenic capacity of immunoglobulins coupled to oligonucleotides or proteins characteristic of autoimmune diseases. (See PCT WO 91/08773). This work is based upon the notion that the induction of tolerance may be strongly influenced by carrier moieties and that immunoglobulin carriers appear to be strongly tolerogenic. Isologous IgG is the preferred carrier, and intravenous administration was the mode used for delivering the conjugates of IgG. Although this body of work extends for more than a decade, oral administration is mentioned only once and only for conjugates where IgA is the immunoglobulin carrier. Thus, although tolerogenic immunoglobulin conjugates are known in the art, such conjugates have never been suggested as agents for inducing a robust response against an antigen characteristic of a pathogen. (To the contrary, the art suggests that such conjugates, if anything, would tolerize a subject against a pathogen which would be highly undesirable). In addition, it never has been suggested that such conjugates would be effective tolerogens when the immunoglobulin is IgG and the mode of delivery is oral delivery.